While the hereinafter described invention was conceived for use in reducing the noise in the cabin of an aircraft, it is to be understood that the invention can be used in other environments and in other types of reinforced skin structures to reduce interior noise and vibration. This includes all types of transportation vehicles--automobiles, buses, trucks, ships, submarines, hovercraft and hydrofoils, for examples. The invention can also be used in the walls and floors of buildings and other enclosures where high noise transmission reduction is desired.
It is to be understood that, because interior noise is reduced by damping the vibrations of a reinforced skin structure, coincidental to the reduction of noise is a corresponding improvement in the sonic fatigue life of the structure and equipment attached to the structure. That is, reducing vibrations not only reduces noise, it also improves the sonic fatigue life of the vibrating structure and attached equipment.
Noise and vibration inside of a reinforced skin structure, such as the cabin of an aircraft, affects passenger speech communication, comfort and sleep. Noise and vibration can also cause structural fatigue and the malfunctioning of equipment mounted in regions of high noise and vibration. Since most transportation vehicles are designed to be as light in weight as possible (commensurate with structural requirements), in order to obtain maximum fuel efficiency, limitations are placed on what designers can do to reduce interior noise and vibration levels. These constraints are particularly severe in the aircraft design field where weight is extremely critical.
In general, noise in an aircraft can be segregated into noise contributing to the overall sound pressure level (OASPL) and noise contributing to the speech interference level (SIL). Typically, the OASPL is essentially determined by the low-frequency content of the noise and the SIL is determined by the mid- to high-frequency content of the noise. Since both the OASPL and the SIL affect passengers, a noise reduction over the entire audio frequency range and, in particular, the low- and mid-audio frequency range is desirable. On the other hand, the whole frequency range is a potential contributor to noise and vibration induced fatigue and malfunctioning of equipment. Even though the following discussion centers on the reduction of interior noise for passenger comfort, it is to be kept in mind that the invention is equally applicable to reducing the detrimental effects of noise and vibration on equipment and structure regardless of whether or not persons or passengers are in the surrounding environment.
Presently, the interior cabinet noise of an aircraft in the mid- and high-frequency range (above 600 Hz) is reduced by applying skin damping tape, lead vinyl sheeting and fiberglass insulation to the walls of the aircraft fuselage. While the use of such items to reduce noise are effective in the mid- and high-frequency range, they are essentially ineffective in the low-frequency range, particularly at frequencies below 300 Hz. Further, they are only moderately effective in the mid-frequency range between 300 and 600 Hz. As a result, the reduction of low- and mid-frequency cabin noise has remained a problem in present-day commercial aircraft.
While both low- and mid-frequency cabin noise remains a problem in commercial aircraft, the problem is acute in recently developed short takeoff and landing (STOL) aircraft, such as externally blown flap (EBF) and upper surface blown (USB) aircraft. The problem is acute in such STOL aircraft because the level of low-frequency interior noise is higher due to the proximity of the engines to the fuselage of the aircraft. As a result, it has now become even more desirable to provide improved methods and apparatus for reducing the OASPL and the SIL in the cabin of an aircraft.
In the past, it was generally believed that cabin noise below about 600 Hz was controlled by the structural stiffness of the fuselage of the aircraft. Thus, attempts to reduce low- and mid-frequency cabin noise were based on various methods of increasing fuselage structural stiffness. For example, in one attempt, the number of stringers in the fuselage of a modern aircraft were doubled to increase the structural stiffness of the fuselage and, thereby, reduce cabin noise. Test data taken on this aircraft indicated that although this 100 percent increase in stringer weight was partially effective in reducing cabin noise in the mid-frequency range (e.g. 300-600 Hz), it was ineffective in the low-frequency range (e.g. below 300 Hz). Thus, although this change improved the subjective impression of the noise level in the cabin of the aircraft, the overall sound pressure level (OASPL) was virtually unaffected.
In recent years, it has been found that during cruise, when pressurization loads cause the skin panel frequency of an aircraft to be higher than the stringer frequency, the coupled mode of the overall reinforced skin structure is such that the skin acts like a very stiff member, supported by relatively flexible stringers. In this regard, attention is directed to U.S. Pat. No. 3,976,269, entitled "Intrinsically Tuned Structural Panel", by Gautam SenGupta. This coupled mode is a very strong radiator of sound because a large section of the skin vibrates in phase. That is, the individual sections of the skin vibrate in phase, whereby vibrations combine to form noise sources having a relatively high magnitude. Since the skin responds like a very stiff member, very little skin flexural bending action takes place. As a result, the application of damping devices (e.g., damping tape), to the skin is not very effective in reducing the low frequency noise produced by such structures. On the other hand, the vibration response of this coupled mode is strongly determined by the deflection of the relatively flexible stringers. As a result, damping the stringers is a very effective way of reducing the low frequency response of the overall structure.
A method and apparatus for significantly reducing the noise produced by stringer response is described in U.S. patent application Ser. No. 029,705, entitled "Method and Apparatus for Reducing Low- to Mid-Frequency Interior Noise", filed Apr. 11, 1979, by Gautam SenGupta and Byron R. Spain. This patent application describes reducing stringer response to vibration disturbances by applying rigid strips along the stringer flanges, the rigid strips being attached to the flanges by thin viscoelastic layers. This method of stringer damping has been found to reduce low-frequency structural vibration and cabin noise during cruise.
While stringer damping using the method and apparatus described in the foregoing patent application is effective in reducing noise when stringer vibration is the dominant noise source, when skin vibration is the dominant noise source, this method is ineffective. In this regard, during takeoff skin vibration is the dominant noise source in most presently designed aircraft. In order to overcome this problem, the foregoing patent application teaches forming the aircraft fuselage such that the fundamental frequency of the skin is higher than the fundamental frequency of the skin-supporting stringers. However, unless this is achieved through cabin pressurization, this approach can lead to an increase in the weight of the aircraft. Alternatively, separate devices can be used to damp skin vibrations. Because the separate devices must cover substantially the entire skin area they add a substantial (and, thus, undesirable) amount of weight.
A method and apparatus for decreasing the amount of weight required when separate damping mechanisms are applied to both the structural reinforcing components and the skin is described in U.S. patent application Ser. No. 079,325, entitled "Method and Apparatus for Wideband Vibration Damping of Reinforced Skin Structures", filed Sept. 27, 1979, by Loyd D. Jacobs, Gautam SenGupta and Byron R. Spain. This patent application describes a method and apparatus for damping the vibration of a reinforced skin structure over a wide frequency range by viscoelastically attaching constraining elements to the skin and to the reinforcing members that support the skin. The constraining elements can be continuous or segmented. The viscoelastic attachment between the constraining elements and the reinforcing members directly damps the vibration of the reinforcing members; and, the viscoelastic attachment between the constraining elements and the skin directly damp the vibration of the skin. Further, the regions of the constraining elements extending between the regions viscoelastically attached to the reinforcing members and to the skin, forms a coupling that allows the skin viscoelastic attachment to indirectly damp the vibrations of the reinforcing member and vice versa. Resonant vibrations due to skin bending, torsional and extensional modes (both cylindrical and panel) and reinforcing member damping, torsional, extensional and tuning fork modes, are all damped.
While apparatus of the type described in U.S. patent application Ser. No. 079,325 reduces vibration and transmitted noise, and improves the sonic fatigue life of the associated structure, the constraining elements that couple the viscoelastic attachments to the reinforcing members and the skin are still add-on components whose only function is to damp the vibrating member. Other than damping, the add-on components do not improve the basic structural design with respect to previous designs. Obviously, it would be desirable to provide vibrational damping mechanisms that also improve other aspects of the structural design, such as thermal transmissibility (i.e., insulation) and manufacturability. The present invention is directed to achieving these results.
It is an object of this invention to provide a reinforced skin structure that includes a mechanism for damping the vibrational response of the reinforced skin structure.
It is another object of this invention to provide a reinforced skin structure that includes a mechanism for damping the vibrational response of the reinforced skin structure that also reduces the thermal transmission through the skin of the reinforced skin structure.